One type of solid state image sensor comprises a substrate body of a semiconductor material having therein a plurality of photodetectors arranged in an array of at least one line. The array may be a single row for a line array, or a plurality of rows and columns for an area array. The photodetectors receive the light from the image and convert the light into charges which are stored in the photodetector. Transfer means, such as a CCD shift register, extends along each line of the photodetectors. The shift register is adapted to receive the charges from the photodetectors and transfer the charges to an output circuit.
One type of photodetector commonly used is a photodiode, and particularly a pinned photodiode. Such a pinned photodiode is described in the article of B.C. Burkey et al. entitled "The Pinned Photodiode for an Interline Transfer CCD Imager", PROCEEDINGS IEEE IEDM, Vol 84, pgs. 28-31, December 1984. The pinned photodiode is used because of its ability to contain large amounts of charge, i.e., large charge capacity. However, certain types of image sensors use photodetectors which are relatively large in size. When pinned photodiodes are used for such large size photodetectors, they provide a charge capacity which is much greater than that which can be handled by the CCD shift register. Thus, the operation of the imager is adversely affected, if, for example, lateral antiblooming is desired. Another problem which can arise with pinned photodiodes results from the fact that the pinned diodes are formed by steps separate from the steps used to form the CCD shift register. Thus process variations which would vary the characteristics of the pinned photodiodes would affect the CCD shift register differently. This could result in photodiodes that have charge capacity that is not well matched to that of the CCD shift register.
Another type of photodetector which has been used in conjunction with CCD shift registers is the photocapacitor. The photocapacitor comprises a lightly conductive region in the body of the image sensor which serves as one plate of the capacitor. A layer of a dielectric material is on the surface of the body and is over the lightly conductive region to serve as the dielectric of the photocapacitor. A layer of a conductive material, such as conductive polycrystalline silicon, is on the dielectric layer and over the lightly conductive region to serve as the other plate of the photocapacitor. Polycrystalline silicon is generally used for the second plate of the photocapacitor since this material is also generally used for the gate electrodes of the CCD shift register. However, this raises a problem since the polycrystalline silicon layer used for the plate of the photocapacitor must be thin, about 500 angstroms, to allow light, especially in the blue region of the spectrum, to pass therethrough. See, for example, the article of C. Anagnostopoulos et al., entitled "Transmittance of Air/SiO.sub.2 /Polysilicon/SiO.sub.2 /Si Structures", published in IEEE JOURNAL OF SOLID-STATE CIRCUITS, June, 1975, pgs. 177-179. However, the polycrystalline silicon layer used for the gate electrodes of the CCD shift register is relatively thick, several thousand angstroms. To deposit a thin layer of the polycrystalline silicon for the photocapacitor and a thicker layer for the CCD shift register gate electrodes complicates the process for making the shift register. Also, it is difficult to connect a terminal to the thin plate layer, which is necessary for the operation of the photocapacitor.